Rotor recentering after decoupling

ABSTRACT

A recentering device for a rotor shaft for recentering a rotor shaft relative to an axis X of a stator structure in the event of decoupling caused by excessive imbalance. The shaft, in normal operating conditions, lies on the axis X and is radially supported by a bearing support disposed in a bore of the axis X in the stator structure. The bearing support has an outside diameter smaller than the diameter of the bore, to enable the bearing support to orbit about the axis X in the event of decoupling. The bearing support is connected to the stator structure by radially fusible elements. The recentering device recenters the bearing support after decoupling.

The invention relates to the difficulty of turbomachines remainingintact in the event of decoupling caused by accidental and excessiveimbalance.

The invention relates more particularly to the integrity of a turbojetafter a fan blade has broken off, for example.

A turbojet comprises an engine which drives a fan that is disposed atthe front of the engine.

The blades of the fan may be damaged following ingestion of foreignbodies, particularly during take off at full revolutions per minute(rpm). Generally, the fan is strong enough to withstand the effects ofingesting foreign bodies without too much damage being caused, and iscapable of continuing to operate, albeit perhaps with reducedefficiency.

In certain circumstances, however, the fan may be damaged to such anextent that it loses pieces of one or more blades. This results in amajor imbalance, which makes it necessary to turn off the engine inorder to reduce danger to the aircraft. Nevertheless, such largeimbalance caused by the loss of a blade leads to major cyclical loadswhich must be accommodated, at least while the engine is running down toreach the windmilling speed of the fan. The windmilling speed is thespeed at which the engine rotates, in a non-operational state, as aresult of the aircraft moving through the air.

A usual way of eliminating the cyclical loads that must be accommodatedby the structure consists in decoupling the rotary shaft of the fan fromthe stator structure at the front bearing of the shaft. This is usuallycarried out by inserting fusible elements between the bearing supportand the stator structure, said fusible elements breaking as soon as theradial forces that must be accommodated by the bearing exceed apredetermined value, i.e. as soon as there is an excessive imbalance.The fan shaft is then free to move radially to a certain extent, and toorbit around the longitudinal axis of symmetry of the engine, and thefan continues to turn about an axis of rotation that passes close to thenew centre of gravity of said fan. Nevertheless, in certaincircumstances, the vibration resulting from the imbalance and thatpersists at the windmilling speed may still be considerable.

This is due to the natural frequency of vibration of the fan and to thereduction in the radial stiffness of the support bearing. Therefore, invarious shaft support arrangements there are means for maintaining acertain amount of bearing stiffness, or even for bringing the axis ofthe shaft substantially back onto the axis of the engine.

Hence, U.S. Pat. No. 6,073,439 provides an annular elastic elementbetween the bearing support and the stator structure, said element beingcoaxial about the axis of the engine, which element exerts radial forceson the bearing support, said radial forces being directed towards theaxis of the engine and tending to bring the axis of the bearing supportback towards the axis of the engine. The stiffness of the linkage afterdecoupling is clearly much less than the stiffness of the bearing in itsnormal operating state without decoupling.

U.S. Pat. No. 6,009,701 describes a fan shaft support bearing in whichthe support of the outer bushing is fastened to the stator structure byradially fusible elements, in order to release the shaft relative to thestator structure in the event of the fusible elements breaking. Thebushing support is surrounded by an open ring of helicoidal shape, whichis capable of cooperating with a conical wall forming part of the statorstructure. Said conical wall includes a helicoidal groove which makes itpossible to move the helicoidal ring from an extreme position where theclearance available for the shaft is the greatest to another extremeposition where the axis of the shaft is once again coaxial on the axisof the engine, after the axis of the shaft has orbited around the axisof the engine while the fan drops in speed from its operating speed tothe windmilling speed.

U.S. Pat. No. 6,009,701 represents the prior art that is closest to theinvention, because the rolling of the open ring in the helicoidal groovedrives a movement in precession of said ring in a direction opposingorbiting of the bearing support axis, and the final positioning of saidring ensures that the bearing is of stiffness that is substantiallyequal to its stiffness in its normal operating state. But thatarrangement requires axial displacement by the open ring on the bearingsupport, and a device to ensure that the open ring is kept stationaryduring normal operation, so that it cannot move at an untimely momentduring normal operation of the engine, which might prevent subsequentdecoupling in the event of excessive imbalance.

The object of the invention is to provide a recentering device which,during normal operating of the engine, cannot prevent subsequentdecoupling, and which ensures satisfactory stiffness of the bearingafter recentering.

The invention thus provides a recentering device for recentering a rotorshaft relative to the axis X of a stator structure in the event ofdecoupling caused by excessive imbalance, said shaft, in normaloperating conditions, lying on the axis X and being radially supportedby a bearing support that is disposed in a bore of axis X in said statorstructure, said bearing support having an outside diameter that issmaller than the diameter of said bore, in order to enable said bearingsupport to orbit about the axis X in the event of decoupling, saidbearing support being connected to the stator structure by radiallyfusible elements, said device comprising means for recentering thebearing support after decoupling.

According to the invention, said recentering device is characterized bythe fact that the recentering means for recentering the bearing supportcomprise means for generating a movement in precession P of said bearingsupport in the direction opposite to the direction of its orbits afterdecoupling, and a plurality of devices for decreasing the clearanceavailable to said bearing support relative to the axis X, said devicesfor decreasing clearance being regularly distributed around the axes ofthe two parts constituted by the stator structure and the bearingsupport, and each part including a first ramp that is provided on one ofsaid two parts and a protuberance provided on the other of said twoparts, said protuberance being, in normal operating conditions, radiallyspaced apart from said first ramp and capable of coming into contactwith said first ramp during the movement in precession of said bearingsupport.

Most advantageously, in order to ensure ideal recentering, all theprotuberances are capable of being in contact with the first ramps atthe same time.

Since the first ramps and the protuberances are stationary elementsfitted respectively to the stator structure and to the bearing support,or on the contrary, to the bearing support and to the stator structure,their positioning in normal operating conditions is defined accuratelyby the positioning of the bearing support on the stator structure duringassembly.

When ideal recentering is obtained, all the protuberances are in contactwith respective first ramps, which ensures bearing stiffness that isclose to the stiffness of the bearing in normal operating conditions.

According to an additional advantageous characteristic, the first ramphas the profile of an involute to a circle, and two adjacent first rampsare connected together by a radial shoulder.

During normal operation, the protuberance is disposed close to theshoulder.

Preferably, the first ramp has the profile of an Archimedes spiral.

In a first configuration, the protuberances are made in the form ofblocks.

In a second configuration, the protuberance is formed by an end portionof a second ramp, said second ramp having a profile similar to theprofile of the first ramp.

In a first embodiment, both the first ramp and the protuberance are madeof metal, and in order to avoid impacts during decoupling, theprotuberance is located, in normal operating conditions, in a positionthat is radially spaced apart from the associated first ramp by adistance that is greater than the expected radial displacement of thebearing support during decoupling.

In a second embodiment, the first ramp is made of elastomer and theprotuberance is made of metal. If the protuberance is a portion of asecond ramp, the distance between the two ramps may be considerably lessthan the expected radial displacement of the bearing support, therebyensuring, in the event of decoupling, that one of the ramps can roll onthe other ramp, so that the precession of the bearing support leads torecentering of the bearing support.

Preferably, the means for generating the movement in precession of thebearing support comprise an elastomer ring secured to the statorstructure, said ring surrounding the bearing support and, being inpermanent contact therewith, so that the bearing support can rollwithout sliding in the bore of said ring after decoupling.

Said elastomer ring is advantageously disposed in the bore of the statorstructure. This makes it possible to eliminate impacts between thebearing support and the stator structure during decoupling.

Other advantages and characteristics of the invention appear on readingthe following description, given by way of example and with reference tothe accompanying drawings, in which:

FIG. 1 is a diagrammatic half-section, based on a radial planecontaining the axis of symmetry of a turbojet, showing the region of thefront bearing that supports the shaft of a fan in normal operatingconditions;

FIG. 2 is a radial section on line II-II in FIG. 1, showing the devicesfor decreasing clearance for the bearing support of an embodiment of theinvention in normal operating conditions;

FIG. 3 is similar to FIG. 2 and shows a second embodiment of theinvention;

FIG. 4 is similar to FIG. 1 and shows the offset of the axis of theshaft from the axis of symmetry of the engine after decoupling;

FIG. 5 shows the disposition of the elements of the devices fordecreasing clearance for the bearing support after recentering followingdecoupling; and

FIGS. 6 and 7 show variant embodiments.

FIG. 1 is a diagram of the front portion of a shaft 1 for driving a fanin a turbojet of axis X, said shaft being held on the axis X in the bore2 of a stator structure 3 by means of a bearing 4 having an insidebushing that is assembled snugly on the periphery of a portion 1 a ofthe shaft 1, and having an outside bushing that is held in the bore of abearing support 5, which bearing support 5 has a diameter that is muchsmaller than the diameter of the bore 2 of the stator structure 3.

The bearing support 5 is connected to the stator structure 3 by radiallyfusible elements, or programmed breaking zones, referenced 6. References7 and 8 indicate axial abutments that are secured to the statorstructure 3 for limiting the axial displacement of the bearing support5.

An annular space 9 is thus arranged radially between the periphery ofthe bearing support 5 and the wall of the stator structure 3 whichdefines the bore 2. The radial thickness of the space is equal to thedifference in diameter between the bore 2 and the outside diameter ofthe bearing support 5, and it is designed to be large enough to allowfor radial displacement of the axis of the bearing support 5, in theevent of the fusible elements 6 breaking, following excessive imbalancecreated by a blade breaking, for example, during normal operation of theturbojet.

As shown in FIG. 1, an elastomer ring 10 that is secured to the statorstructure 3 is disposed in the annular space 9. Said elastomer ringsurrounds the bearing support 5 and includes an inside bore having adiameter that is slightly larger than the diameter of the bearingsupport 5.

When the fusible elements 6 break, as shown in FIG. 4, the axis 11 ofthe shaft 1, which is also the axis of the bearing support 5, moves awayfrom the axis X of the stator structure 3, and the bearing support 5comes to bear against the inside surface of the bore in the elastomerring 10.

The axis 11 begins to orbit about the axis X in the direction ofrotation R of the shaft 1. As a result, the bearing support 5 begins toroll, preferably without sliding, in the bore inside the elastomer ring10. Said rolling movement causes the bearing support 5 to move inprecession P in a direction which is opposite to the direction ofrotation R, as in an epicyclic gear system, at a speed that is afunction of the diameters of the bearing support 5 and of the bore inthe elastomer ring.

The proposed bearing arrangement also comprises devices 20 fordecreasing clearance relative to the axis X, after decoupling, saiddevices being implemented by the movement in precession P.

Such devices 20, which are three in number in the examples shown inFIGS. 2 and 3, are regularly distributed around the axis X, and eachcomprise a first ramp 21, preferably having the profile of an involuteto a circle or of an Archimedes spiral, that is provided on one of theparts constituted by the stator structure 3 and the bearing support 5,and a protuberance 22 that is axially offset from the first ramp 21, theprotuberance being provided on the other one of the parts, constitutedby the bearing support 5 and the stator structure 3. Two adjacent ramps21 are connected by a radial shoulder 23.

In normal operating conditions, i.e. in the absence of decoupling, theprotuberance 22 is disposed circumferentially in the vicinity of aradial shoulder 23 and the distance that separates the protuberance 22from the first associated ramp 21 is greater than the radialdisplacement expected of the bearing support 5 immediately afterdecoupling, in order to prevent a protuberance 22 and the firstassociated ramp 21 from impacting each other when decoupling occurs,with the radial forces generated by decoupling then being taken up bythe elastomer ring 10.

During the movement in precession P of the bearing support 5 afterdecoupling, the protuberances 22 move away from the associated shoulders23. Given the profiles of the first ramps 21, the minimum clearanceexisting between the tips of the protuberances 22 and the first ramps 21will gradually decrease until one of them comes into contact with thefirst associated ramp 21. From then on, the clearance available for thebearing support 5 is limited by said successive contacts, and the axis11 of the bearing support 5 moves closer to the axis X during subsequentmovement in precession P by the bearing support 5.

When the three protuberances 22 are in contact at the same time with thethree first ramps 21, the axis 11 coincides with the axis X, therebyachieving ideal recentering. The profile of the first ramps 21 isdesigned to enable this condition to be achieved, by selecting thedistance lying between the tips of the protuberances 22 and the axis ofsymmetry, X or 11, of the part that carries them, so that said distancelies between the distances of the ends of the radial shoulders 23 fromthe axis of symmetry 11 or X, of the part that carries them.

In FIG. 2, JB indicates clearance for the expected displacement of theaxis 11 of the bearing support 5, during decoupling, and JS indicatesthe safety clearance needed to avoid impacts when decoupling takesplace. At the beginning of the movement in precession P, the clearanceJS decreases without there being any contact between the protuberances22 and the first ramps 21. Then as the clearance JB is taken up,successive contacts are made between the protuberances 22 and the firstramps 21. Once the clearance JB has been taken up in full, as shown inFIG. 5, the bearing support 5 is recentered on the axis X.

FIG. 3 shows the protuberances 22 made in the form of blocks secured tothe bearing support 5, the first ramps 21 being secured to the statorstructure 3.

FIG. 2 shows the protuberances 22 constituted by the end portions of aset of second ramps 24, said ramps having profiles identical to theprofiles of the first ramps 21.

FIG. 5 shows the position of the ramps 21 and 24 in FIG. 2 when thethree end portions 22 of the second ramps 24 are in contact with thefirst ramps 21 at the same time, which position corresponds to idealrecentering of the bearing support 5 and, hence, of the shaft 1.

The bearing support 5 is then immobilized and ceases to rotate by awedge effect between the end portions 22 and the first ramps 21.

The ramps 21 and 24 are preferably made of metal and can be disposed inthe annular space 9, as shown in FIG. 1.

However, the first ramps 21 could also be provided on the axial abutment8, with the second ramps 24 or the blocks being provided on the outsidebushing 4 a of the bearing 4, outside the annular space 9, as shown inFIGS. 6 and 7.

FIG. 6 also shows a ring 10 made in the form of a flexible elastomerbushing.

As shown in FIG. 7, the ring 10 may be a rigid hoop connected to thestator structure 3 by a flexible metal support 30 that is situatedoutside the annular space 9. What matters, is that the bearing support 5should be permanently in contact with the ring 10 after decoupling inorder to allow for the movement in precession P, at a speed that isproportional to the gear reduction ratio in order to cause the safetyclearance JS and the clearance JB for the expected displacement to betaken up progressively when the first ramp 21 and the protuberance 22move towards each other.

In the embodiments shown in the drawings, the three protuberances 22 canall be in contact with the three ramps 21 at the same time, therebyensuring ideal recentering. It should be observed that it is possible toprovide a device to limit the maximum angle of precession of the bearingsupport 5 relative to the stator structure 3, in order to make sure thatthe protuberances 22 are positioned quite near the first ramps 21 aftera predetermined movement in precession by the bearing support 5, whichcorresponds to residual clearance of small amplitude.

Another variation of the device proposed consists in replacing at leastone of the metal ramps 21 and 24 by elastomer ramps. The only drawbackto such a solution is that smaller stiffness is obtained afterrecentering, but then it would no longer be necessary to use anelastomer ring 10 nor a flexible metal support 30, such as the supportsshown in FIGS. 6 and 7, since elastomer ramps also perform the functionof moving the bearing support 5 in precession in order to take up theclearance.

1-13. (canceled)
 14. A recentering device for a rotor shaft forrecentering a rotor shaft relative to an axis X of a stator structure inan event of decoupling caused by excessive imbalance, said shaft, innormal operating conditions, lying on the axis X and being radiallysupported by a bearing support disposed in a bore of the axis X in saidstator structure, said bearing support having an outside diameter thatis smaller than a diameter of said bore, to enable said bearing supportto orbit about the axis X in the event of decoupling, said bearingsupport being connected to the stator structure by radially fusibleelements, said device comprising: means for recentering the bearingsupport after decoupling, wherein the recentering means of the bearingsupport comprises means for generating a movement in precession by saidbearing support in a direction contrary to a direction of its orbitstraveled after decoupling, and a plurality of devices for decreasingpermitted clearance of said bearing support relative to the axis X, saiddevices for decreasing clearance being arranged regularly around theaxes of the two parts constituted by the stator structure and thebearing support, and each part including a first ramp provided on one ofsaid two parts and a protuberance provided on the other of said twoparts, said protuberance being, in normal operating conditions, radiallyspaced apart apart from said first ramp and configured to come intocontact with said first ramp during the movement in precession of saidbearing support.
 15. A device according to claim 14, wherein all theprotuberances are configured to be in contact with the first ramps at asame time.
 16. A device according to claim 14, wherein each first ramphas a profile of an involute to a circle, and two adjacent first rampsare connected by a radial shoulder.
 17. A device according to claim 16,wherein each first ramp has a profile of an Archimedes spiral.
 18. Adevice according to claim 14, wherein the protuberances are made in aform of blocks.
 19. A device according to claim 14, wherein eachprotuberance is formed by an end portion of a second ramp, said secondramp having a profile similar to a profile of the associated first ramp.20. A device according to claim 14, wherein each first ramp and theprotuberance are made of metal.
 21. A device according to claim 20,wherein each protuberance is located, in normal operating conditions, ina position radially spaced apart from the associated first ramp by adistance that is greater than an expected radial displacement of thebearing support during decoupling.
 22. A device according to claim 19,wherein each first ramp is made of elastomer and the associated secondramp is made of metal and can roll on the first ramp, without sliding,after decoupling, to generate the movement in precession.
 23. A deviceaccording to claim 14, wherein the means for generating the movement inprecession comprises an elastomer ring secured to the stator structure,said ring surrounding the bearing support and being in permanent contacttherewith so that the bearing support can roll without sliding in thebore of said ring after decoupling.
 24. A device according to claim 23,wherein said ring in elastomer is disposed in the bore of the statorstructure.
 25. A device according to claim 23, wherein said ring isrigid and is connected to the stator structure by a flexible metalsupport.
 26. A device according to claim 14, comprising three firstramps and three protuberances.